146 METALLIC STAINS (IMPREGNATION METHODS). 



(one volume of water to one of the acid of 1'12 sp. gr.), and 

 remain there until the epidermis peels off. They then are 

 put for fifteen minutes into gold-chloride solution (1J per 

 cent, to 1 per cent.), then for twenty-four hours into dilute 

 formic acid (1 part of the acid to 1 3 of water), and then for 

 twenty-four hours into undiluted formic acid. (Both of these 

 stages are gone through in the dark.) Thin sections are then 

 made and mounted in dammar or glycerin. Successful prepa- 

 rations show the nerves alone stained, but it is not possible 

 always to control the results. 



225. Ranvier's Formic Acid Method *(Q?mr. Journ. Mic. Sci. 

 [N.S.], Ixxx, 1880, p. 456). The method of Lowit has been 

 modified by many workers by omitting the final treatment 

 with undiluted formic acid, and also in some other details. 

 Ranvier proceeds as follows : Reflecting that the action of 

 the one third formic acid in which Lowit placed his tissues 

 must be hurtful to the final ramifications of the nerves, he 

 combines the formic acid with a fixing agent designed to 

 antagonise its altering action, and takes for this purpose the 

 chloride of gold itself. The tissues are placed in a mixture 

 of chloride of gold and formic acid (4 parts of 1 per cent, gold 

 chloride to 1 part of formic acid) which has been boiled and 

 allowed to cool (Ranvier's Traite, p. 826). They remain in 

 this until thoroughly impregnated (muscle twenty minutes, 

 epidermis two to four hours) ; the reduction of the gold is 

 effected either by the action of daylight in acidulated water, 

 or in the dark in dilute formic acid (1 part of the acid to 

 4 parts of water). 



The object of boiling the mixture of gold chloride and 

 formic acid is this, that " by boiling in the presence of the 

 acid the gold acquires a great tendency to reduction, and 

 for this reason its selective action on nervous tissues is 

 enhanced/' 



226. Eanvier's Lemon-juice Method (Traite, p. 813). Instead 

 of combining the formic acid with gold chloride in order to 

 mitigate its action, recourse may be had to a less injurious 

 acid than formic acid. Ranvier finds that of all acids lemon- 

 juice is the least hurtful to nerve-endings. He therefore soaks 

 pieces of tissue in fresh lemon- juice, filtered through flannel, 

 until they become transparent (five or ten minutes in the case 



